Flame spraying involves the heat-softening or melting of a heat-fusible material, such as metal or ceramic, and the propelling or spraying of the softened material against the surface to be coated. Flame spraying is effected utilizing a flame spray gun. In certain types of guns the heat-fusible material is initially fed through the gun in powdered form. Guns which are so supplied with material to be sprayed in powdered form are known as "powder-type flame spray guns." The powdered material is generally known as a flame spray powder and is usually of a relatively small particle size, as, for example, below about 140 mesh U.S. Standard screen size. The flame spray guns utilize a combustion or plasma flame and in many guns the powder is fed to the gun or into the gun's flame entrained in a carrier gas. In order to obtain high quality coatings it is necessary to accurately control the rate of powder fed through the gun and to maintain a constant rate for given spray conditions. Powder, and particularly the fine powder of the type used as flame spray powder, has proven to be a difficult material to handle and feed in a carrier gas at the desired uniform rate.
Powder feeders such as described in U.S. Pat. Nos. 3,138,298; 3,501,097 and 3,976,332 utilize metering devices for conveying powder from a hopper into a powder feed hose at a uniform feed rate. The metering systems of different designs and modes of operation utilize gravity, mechanical and gas conveyances and combinations thereof. Typically the metering device or hopper has attached thereto a vibrator to maintain the powder in loose free-flowing form. The feed hose between the powder feed device and the flame spray gun is a flexible conduit typically of durable, fiber-reinforced plastic material to allow movement and manipulation of the flame spray gun with respect to the workpiece being coated. The manipulation of the gun may be by hand or by an automatic robot-type machine.
In spite of the precision powder feeding available from devices such as described in the above-mentioned patents, considerable variation is encountered in the actual feed rate. Pulsing and surging often occur in the powder flow at the output of the flame spray gun. This often occurs when the feeder has been turned off and back on again. Pulsing and surging also occurs during spraying.
It has been found that these difficulties are often related to settling and packing of powder in the feed hose. The feed hose generally sags in the middle, providing a low region in which powder particles that drop out of the carrier gas can accumulate. When spraying is stopped, the powder clogs the hose causing blockage. On restarting a clump of powder may issue from the gun. It is theorized that pulsing during spraying may be due to a cyclical phenomenon of powder settling out of and being re-entrained in the carrier gas in the feed hose. A similar effect is noted when the flame spray gun is moved, apparently causing a relocation of the low region of the feed hose and thereby a change in the settling of the powder.
Therefore, it is the primary objective of the present invention to provide a powder feed system for a flame spray gun which will reliably feed powder at a uniform rate without the above-mentioned difficulties.
It is a further objective of the present invention to provide a powder feed system to convey powder therethrough at a constant flow rate.
It is still a further objective of the present invention to provide a powder feed system in which powder will not disadvantageously settle.